Regarding sales as opposed to usage:

Windows once held an overwhelming lead in 2008 with about 70 percent of the OS market, Forrester found in estimating overall personal computing device sales by OS. By 2012, that market share had shrunk to about 30 percent.

-- http://rcpmag.com/articles/2012/10/22/microsoft-os-market-sl...

According to a Goldman Sachs report acquired by The Seattle Times, Windows lost massive share in the global compute market-- from being the OS of choice in 97% of all computing devices in 2000 to just 20% in 2012.

-- http://www.consumerit.eu/index.php?option=com_content&vi...